Fused salt electrolysis cell



Jan. 21, 19 s. E. ECKERT ETAL FUSED SALT ELECTROLYSIS CELL 2 Sheets-Sheet 1 Filed May 5, 1961 FIG.1

6 I6 I Fl 6.18 M

FIGlA INVENTORS STANLEY E. EOKERT Q FRANCIS J. ROSS BYWxSQW M101,

AGENT Jan. 21, 1964 s. E. ECKERT ETAL 3,118,827

FUSED SALT ELECTROLYSIS CELL Filed May 5, 1961 2 Sheets-Sheet 2 FIG.2

INVENTORS STANLEY E. ECKERT FRANCIS J. ROSS BY MM AGENT United States Patent Gfilice mute,

3,118,827 FUSED SALT ELECTROLYSIS CELL Stanley E. 'Eckert and Francis J. Ross, Niagara Falls,

N.Y., assignors to E. I. du Pont de Nemours and Company, Wilmington, Dei., a corporation of Delaware Filed May 5, 1961, Ser. No. 108,039 3 Claims. (6!. 204-247) This invention relates to a fused salt electrolytic cell for the production of a molten metal having a density below the density of the molten electrolyte and more particular-1y to a cell for the production of sodium involving a plurality of anode and cathode elements.

The electrolysis or" a fused salt bath in a Downs type sodium cell as originally described in US. Patent 1,501,756 (July 15, 1924), requires the use of a permeable diaphragm to separate anode and cathode products. This diaphragm is usually a cylindrical Wire mesh screen surrounding the submerged vertically disposed cylindrical graphite anode. Since the bath olfers considerable electrical resistance, it is necessary to keep the distance between anode and cathode as small as possible, usually below 3 inches. In order to operate at reasonable temperatures, it is preferable to employ a bath consisting of a mixture of fused salts, principally the chlorides of sodium and alkaline earth metals, especially calcium. Operation of such a cell results in the formation of sodium at the cathode together with small amounts of alkaline earth metal, especially calcium, as well as smaller amounts of other impurities, usually solid, originating from trace contaminants. These impurities, principally electrically conductive solids at cell bath temperatures, have been found to accumulate in various areas between the diaphragm and the cathode thereby causing electrical shorts which often damage the diaphram. In addition, the solid deposits lead to loss of production which is directly proportional to the extent of cell malfunction caused by short circuiting and the shutdown time necessary to correct difficulties arising therefrom.

Calcium deposition can obviously result from the development of high local concentrations of calcium salts. This is particularly likely in the narrow catholyte zone between cathode and diaphragm where sodium is preferentially removed from the electrolyte. Accordingly, since natural diffusion in early fused salt sodium cells did not provide adequate electrolyte circulation, various methods were developed for improving this situation, Gilbert, U.S. Patent 2,111,264 (March 15, 1938), employed air-lift pumps and cathode perforations to provide a positive means of causing the catholyte to circulate downwardly between the active surface of the cathode and the diaphragm. McNitt, US. Patent 2,315,443 (March 30, 1943), and 2,390,114 (December 4, 1945), accelerated circulation in the cathode zone by creating temperature differentials in the cell. Anolyte circulators, such as that of Hulse et al., US. Patent 2,194,444 (March 19, 1940), were employed principally to prevent freezing of electro lyte in the upper part of the cell and to assist in chlorine removal. However, although the aforesaid catholyte circulating devices prevented excessive calcium deposition and were of some value in cells containing a single cathode-anode element, short circuiting due to solid deposition has remained a problem. This problem has become more serious with the development of the modern high capacity cells containing a plurality of anode-cathode elements and complicated diaphragm-collector assemblies. Recent means for solving this problem have involved physical techniques for shaking the entire collector assembly and special methods for controlled movement of the diaphragms alone, Gal'linger, US. Patent 2,924,558 (February 9, 1960).

It is an object of the present invention to reduce deposition of conductive solids in the electrolysis zone of fused salt cells, containing at least two anode-cathode elements. it is also an object to improve diaphragm life and current efficiency of fused salt sodium cells. It is an additional object to reduce depletion of sodium ions or enrichment of calcium ions in the catholyte zone by providing an accelerated circulation of fused electrolyte. It is still a further object to provide an improved collectorassembly for the aforesaid sodium cells with means for enhancing catholyte circulation. These and other objects are achieved according to the present invention as described in detail hereafter.

It has been discovered that the above objects may be accomplished by incorporating a circulator conduit in the collector-assembly of a fused salt cell disposed in such fashion as to permit electrolyte from the anodic product collector to pass downward into the catholyte zone. The desired results are obtained in a fused salt sodium cell containing at least two and preferably four cathode-anode elements by use of a vertically disposed circulating conduit. This conduit allows substantially gas-free electrolyte from a point somewhat above the roof of the sodium collecting channel in the chlorine collector dome to flow downward to a point which is either at, or below, the electrolyte-molten sodium interface in the catholyte zone. Preferred results are obtained when the circulator discharges in the electrolytemolten sodium interface area, but the invention also functions when the circulator extends downward to a point not lower than the bottom of the cathode.

The drawings illustrate in schematic fashion a preferred embodiment of the invention and several alternates.

FIGURE 1 is a vertical cross-section of a sodium cell having four anode-cathode elements showing the collector assembly and the catholyte circulator conduit. This section is at II of FIGURES 2 and 3.

FIGURES 1A and 1B are fragmentary vertical sections of alternate circulators drawn on the same scale as FIG- URE 1.

FIGURE 2 is a horizontal section showing the collector assembly at Ii-II of FIGURE 1.

FIGURE 3 is a horizontal section of the cell of FIG- URE 1 at llIIi'I.

As illustrated in the drawings, the sodium cell comprises a steel shell or containing vessel 12 lined with a refractory ll. symmetrically arranged therein are four cylindrical graphite anodes 7 7 '7 and 7 A steel cathode assembly comprises four cylindrical cathodes, 9 9 9 and 9 and the supporting cathode arms 10 and 101 The approximate level of arms 10 and 10 is indicated schematically in FIGURE 1 by dotted lines.

The cylindrical anode-cathode elements comprise a central graphite or carbon anode and a surrounding steel cathode. Cylindrical permeable diaphragms 3 8 8 and 8 are supported substantially midway in the electrolytic zone between anode and cathode. In the operating cell, these anode-cathode elements are immersed in fused electrolyte and the diaphragms separate the central anode products from the peripheral cathode products and define the borders of the anolyte and catholyte zones. The collector assembly is mounted above the electrode elements and comprises a molten sodium collector channel 5 having an upper roof 6 pierced by cylindrical diaphragm supporting chimneys 2 2 2 and 2 Surmounting sodium channel roof 6 is the anodic product dome 4 which receives the chlorine charged anollyte ascending through the diaphragm supporting chimneys. Sodium leaves the collector channel 5 by ascending riser pipe 3 through which it is removed from the cell. Normally, the roof of the sodium channel slopes upward to the riser pipe to assure smooth flow of sodium thereto, but this is not shown in the schematic drawings. The catholyte circulator conduit 1 is usually :located in the center of the dome and is vertically disposed so that it passes downward through the horizontal axis of the roof 6 of the sodium collector channel 5. It preferably extends from or somewhat above the roof of the sodium channel but below the level of the molten salt electrolyte 14 to a point somewhat above the level of the lower edges of the diaphragm-supporting chimneys 2 and is so disposed that it is about one to six inches below the molten electrolyte-metal interface 15 in the sodium collector channel. The conduit is preferably a cylindrical pipe with a capped bottom 17 whose vertical Wall is pierced with at least one and usually four exit ports 16 as indicated in the schematic drawing of FIGURE 1. Modified circulator, FIGURE lA, differs from the circulator of FIGURE 1 only in that it descends into the catholyte area to a point above the level of the cathode bottoms where its vertical wall is pierced with one or more exit ports 16 just above the capped bottom 17. FIGURE 1B shows another alternate deep catholyte circulator having exit ports 16 similar to 1 above the level of the lower edge of the diaphragm-supporting chimneys, means for restricting the flow of electrolyte consisting of a constriction or bafile 18 just below these ports as well as lower exit ports 16 located just above the level of the bottom of the conduit 17. The bottom of the circulator conduit of FIGURE 13 may be located at any point between the level of the vertical midpoint of the cathode and the cathode bottoms. Although side exit ports in the circulator conduit as shown in the drawings are preferred, the circulator exit may also take the form of an open bottom to the conduit.

As previously noted, during electrolysis, the cell is substantially filled with the molten salt electrolyte. Anolyte circulation indicated by arrows in FIGURE 1 is assisted by upward flow of chlorine gas bubbles from the anodes 7 thru the diaphragm-supporting chimneys 2 into the chlorine collector dome 4 where chlorine escapes through conduit 19. This movement raises the molten salt level 14 in the chlorine dome above the cell level 13. Catholyte circulation also indicated by arrows is assisted by the normal upward flow of molten sodium from the active inner surface of the cylindrical cathodes 9. Down flow of catholyte normally takes place by natural diffusion around the outer inactive surface of the cathodes. The catholyte circulator of this invention increases catholyte circulation by allowing an effective stream of electrolyte from the chlorine collector dome 4 to flow downward into the central catholyte zone. Prior to the development of the catholyte circulator of this invention, anolyte in the chlorine collector zone had substantially no opportunity to circulate.

Circulation rates through the catholyte circulator of this invention influence the results obtained and should be adjusted to secure optimum results for a given cell. The rate of circulation is controlled in part by the diameter and length of the conduit. It may also be modified for a given conduit size by adjusting the height of the circulator pipe above the roof of the sodium collector channel within the zone of the hydraulic head difference between the electrolyte level inside the chlorine collecting dome 14 and the level outside the dome 13. This differ- .ence may be in the range 4 to 6 inches depending on cell design and operating amperage. In addition, the circulation rate may also be adjusted by restricting the discharge of the circulator by controlling the size of outlet orifices 16 and the baifie 18. When the circulator outlet is just below the sodium-electrolyte interface 15, the one or more discharge orifices are preferably designed to direct the discharged electrolyte flow parallel with and just below the molten metal interface boundary. The circulating current is fundamentally a result of gravity, the natural circulation of the cell and density differences due to normal temperature variations. No impeller is employed.

Sodium cells equipped .with the novel catholyte circulator of this invention show reduced depletion of sodium ion in the catholyte zone since they provide an accelerated circulation of fresh bath. These cells also show a decrease in calcium deposition and a consequent increase of diaphragm life. This reduces both the expense of diaphragm replacement and loss of production during the replacement process. In addition, cells with the catholyte circulator show an improvement in current efficiency over that obtained with similar cells without the circulator. It has been demonstrated that unusually good results are obtained with the preferred circulator alternate in which the exit ports from the bottom of the conduit direct flow in the plane of sodium-electrolyte interface and are located just below this interface as shown in FIGURE 1. In the case of sodium cells containing 4 anode-cathode elements, it was found that an average current efficiency of 75 increased to that of -85% when the interface directed circulator was installed. In addition, this change increased the average cell diaphragm life from 11 to 26 days. The electrolyte employed in the cell bath was the usual sodium chloridecalcium chloride mixture commonly used in Downs-type cells. A comparable degree of improvement was also obtained with ternary baths containing barium chloride in addition to the aforesaid constituents by measuring behavior before and after installation of the catholyte circulator.

It should be noted that for optimum results, electrolyte circulation must not be excessive. In general, circulation through the catholyte circulator should be in the magnitude of about one to sixty gallons per minute. When the circulator outlet is just below the electrolytemetal interface, the preferred rate is one to twenty gallons per minute. For a deep circulator as shown in FIG- URE 1A, the preferred rate is in the range 10 to 60 gallons per minute. In general, the optimum circulation rate will depend on the type and size of the sodium cell and may be readily determined by experiment.

The disadvantage inherent in excessive circulation may be due to entrainment of products from or into the catholyte area, diaphragm leakage, and other purely physical phenomena which would result in allowing sodium and chlorine to interact and thus reduce cell efficiency.

The catholyte circulator is not limited to the four-anode fused salt sodium cell which has been used to exemplify the invention. It may be used with cells having two or three anode-cathode elements or with cells having a larger number of electrode pairs. In the latter cells, it may be desirable to employ several catholyte circulator conduits to facilitate circulation.

This invention is generally applicable to electrolytic cells involving the electrolysis of fused salt m'stures wh re the metal product of the electrolysis will float on the cell bath and both the anodic and cathodic products are collected at the top of the cell.

We claim:

1. In a fused salt Downs-type electrolytic cell for the production or" a molten metal having a density lower than that of the fused salt electrolyte and a gaseous anode product, said cell comprising (a) at least two vertically disposed anode-cathode assembiies, each comprising an anode, a cathode and a diaphragm disposed therebetwecn and separating the electrolyte into anolyte and catholytc zones; and

(/2) a collector assembly mounted above said anodecathode assemblies comprising a submerged cathode product collector, a surmounting anode product collector and chimneys leading upwards from the anolyte zones of the cell and opening into said anode product collector, said chimneys defining vertical passageways for the upward flow of electrolyte and anode product from said anode zones into said anode product collector, said cathode product collector comprising a generally horizontally disposed roof wherein the conduit to permit flow of electrolyte from 10 the anode product collector into the catholyte zone is provided with a discharge for the electrolyte at about the level of the upper ends of the cathodes.

3. In a cell according to claim 1, the improvement wherein the conduit to permit flow of electrolyte from the anode product collector into the catholyte Zone is provided with a discharge for the electrolyte at about the 5 level of the lower ends of the cathodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,924,558 Gallinger Feb. 9, 1960 FOREIGN PATENTS 516,775 Great Britain Jan. 11, 1940 

1. IN A FUSED SALT "DOWNS"-TYPE ELECTROLYTIC CELL FOR THE PRODUCTIN OF A MOLTEN METAL HAVING A DENSITY LOWER THAN THAT OF THE FUSED SALT ELECTROLYTE AND A GASEOUS ANODE PRODUCT, SAID CELL COMPRISING (A) AT LEAST TWO VERTICALLY DISPOSED ANODE-CATHODE ASSEMBLIES, EACH COMPRISING AN ANODE, A CATHODE AND A DIAPHRAGM DISPOSED THEREBETWEEN AND SEPARATING THE ELECTROLYTE INTO ANOLYTE AND CATHOLYTE ZONES; AND (B) A COLLECTOR ASSEMBLY MOUNTED ABOVE SAID AONCECATHODE ASSEMBILIES COMPRISING A SUBMERGED CATHODE PRODUCT COLLECTOR, A SURMOUNTING ANODE PRODUCT COLLECTOR AND CHIMNEYS LEADING UPWARDS FROM THE ANOLYTE ZONES OF THE CELL AND OPENING INTO SAID ANODE PRODUCT COLLECTOR, SAND CHIMNEYS DEFINING VERTICAL PASSAGEWAYS FOR THE UPWARD FLOW OF ELECTROLYTE AND ANODE PRODUCT FROM SAID ANODE ZONES INTO SAID ANODE PRODUCT COLLECTOR, SAID CATHODE PRODUCT COLLECTOR COMPRISING A GENERALLY HORIZONTALLY DISPOSED ROOF HAVING A DOWNWARDLY DEPENDING FLANGE ABOUT ITS PERIPHERY, WHICH ROOF IS PIERCED BY SAID CHIMNEYS, THE IMPROVEMENT COMPRISING A CONDUIT TO PERMIT FLOW OF ELECTROLYTE DIRECTLY FROM SAID ANODE PRODUCT COLLECTOR INTO SAID CATHLYTE ZONE, ASIAD CONDUIT PIERCING SAID ROOF OF THE CATHODE PRODUCT COLLECTOR AND LEADING FROM WITHIN SAID ANODE PRODUCT COLLECTOR DOWNWARDLY INTO SAID CATHOLYTE ZONE. 